Direct current machines, i.e., motors or generators, have a commutator fixed to the rotating armature and electrically connected to the armature windings, the armature being electrically connected to an external power circuit through brushes which engage the rotating commutator. The brushes are usually made of a mixture of carbon particles and a binder material such as graphite or a metallic powder. As the commutator rotates, the contact faces of the brushes gradually wear away. In order to maintain electrical contact the brushes are slidably mounted in stationary brush holders on a yoke surrounding the commutator, the brushes being spring-pressed into engagement with the commutator. Wearing of the brushes causes them to shorten in length and eventually they must be replaced. If a brush is allowed to wear too much before replacement, the metal securement of the pigtail connection to the brush will engage the commutator and cause detrimental scoring thereof.
It is the object of this invention to provide an improved means of detecting the amount of wear of a brush and for giving a warning signal when a predetermined amount of brush wear has occurred.